1. Field of the Invention
This invention relates to an extrusion die and in particular to a helical gear extrusion die.
2. Description of the Prior Art
In the process of forming articles by extruding metal through a die having the contour of the part to be formed, it is known that the extrusion expands radially outward after being forced through the forming surface of the die. The radial expansion is a result of elastic deformation of the extrusion blank when the blank is forced through the forming throat of the die. The strain energy stored in the blank is released after the blank passes the forming surface of the die. As the blank is forced through the die past the forming land, enormous pressures are required in order to reshape the cylindrical outer surface of the extrusion blank into a fluted surface having helical teeth. The metal blank flows around the end faces of the teeth of the die and past the extrusion land where the form of the teeth and the size and direction of the helix angle are established. Enormous pressure is required in the process of forcing the blank through this constricted space and large stresses are developed in the teeth of the die, particularly in the region of its end face where the extrusion blank undergoes the most extreme dimensional change.
The extrusion process is facilitated by lubricating the extrusion blank so that it passes more readily through the constricted orifice of the die. In the prior art, such as that discussed in U.S. Pat. Nos. 3,910,091 and 4,287,749, the end faces of the die teeth, at the die orifice, are substantially planar although inclined in the direction that the blank passes through the die. This inclination produces a lead or transition surface having a component in the axial direction.
It is preferred that an extrusion die for forming helical gears also have, in addition to a lead surface tending to direct the die blank material in the radial direction, a transition or lead surface directed circumferentially with respect to the axis of the die.
Furthermore, an extrusion die for this purpose having lead surfaces that facilitate the passage of the die blank through the die orifice must provide sufficient strength of the die teeth in relation to the compression, bending, tension and shear stresses developed in the die teeth in the vicinity of the forming orifice.